1. Field of the Invention
The present invention relates to ink jet printheads and more particularly to multicolor ink jet printheads for use in continuous stream type ink jet printing systems.
2. Description of the Prior Art
Generally, in ink jet devices employing multiple nozzles from which continuous streams of ink droplets are admitted and directed to a recording medium or a collecting gutter, there are one or more rows of nozzles which receive an electrically conductive recording fluid, hereinafter called ink, from a pressurized supply reservoir or manifold. The ink may be, for example, a water-base fluid, and is ejected through the nozzles in rows of parallel streams or filaments. The ink is stimulated prior to or during its exiting from the nozzles so that the stream breaks up into a series of uniform droplets at a fixed distance from the nozzles. As the droplets are formed, they are selectively charged by the application of a charging voltage to charging electrodes positioned adjacent the streams at a location where they break up into droplets. The droplets which are charged, are deflected by an electrical field, either into a gutter for ink collection and reuse, or to a specific location on a recording medium, such as paper, which may be continuously transported at a relatively high speed across the droplets.
Printing information is transferred to the droplets through charging by the charging electrodes. The charging control voltages are applied to the charging electrodes at the same frequency as that which the droplets are generated. This permits each droplet to be individually charged so that it may be positioned at a distinct location different from all other droplets or sent to the gutter. Printing information cannot be transferred to the droplets properly unless each charging electrode is activated in phase with the droplet formation at the associated ink stream. Failure to do this results in partially charged droplets which may miss the gutter or deposit the droplet at an erroneous position on the recording medium.
It is therefore apparent that ink jet droplet printers of the continuous stream type described above cannot be operated at their maximum quality level unless the droplets in all streams are charged in synchronism with their associated generation morphology. This implies either a measurement of droplet generation timing for each and every stream, or control of the droplet generation in such a way that the timing or phase of droplet generation is predetermined. The latter is not practically achieved for high quality printing. Consequently, the correct phase is determined during calibration for each jet utilizing droplet sensors.
A variety of printhead designs have evolved for single color, continuous stream ink jet printing. While color printing with ink jets has been considered a strong point for the technology, design proposals and prior art printheads for the implementation of multicolored, continuous stream type printing have been relatively embryonic up to this point in time. For example, each color has been considered to be applied by a separate drop generator and charge/deflection/guttering structure. Consequently, the design is burdened by tight mechnical alignment tolerances between the printheads for quality printing and duplication of hardware elements, both resulting in expensive manufacturing costs. The present invention is directed towards identifying means whereby a unitized multicolor printhead may be designed to minimize duplication of hardware elements and have the tight mechanical alignment tolerances relegated to an inherently precise manufacturing process.
U.S. Pat. No. 4,320,406 to Heinzl discloses an ink printing device for a piezoelectrically driven multicolored printhead of the drop-on-demand type. The printhead comprises a plurality of ink passageways or channels, each with its own piezoelectric driver. The passageways serve as ink channels and terminate at a discharge end that abuts restricted orifices formed in a plate. Ink droplets are ejected through the orifices in paths perpendicular to the recording medium. Corresponding ink channels extend rearward from the orifice plate in diverging patterns. The rear end of each channel communicates with a corresponding restricted passage formed transversely through a throttle plate. Each ink channel is concentrically surrounded therealong by a separate piezoelectric drive element. Upon application of a voltage to one or more of the drive elements, a channel portion is contracted to generate a pulse which causes ejection of a droplet from the orifice, which is propelled toward the recording medium. Ink is supplied to the ink channels via a one of a plurality of ink manifolds formed in the rear of the printhead body. Each manifold chamber communicates respectively with a particular set of ink channels. In the preferred embodiment, respective ink channel sets are connected through the throttle plate with its associated manifold chamber. The manifold chambers are supplied with ink via corresponding supply lines which are in fluid communication with corresponding individual ink reservoirs, each having a different color of ink.
Japanese patent application No. 54-134226, published without examination on May 22, 1981, as laid open number 56-58876 to Ishihara, discloses an apparent drop-on-demand multicolor recording head comprising a plurality of individual ink jet printheads arranged radially about a point on the recording medium so that the ink jet heads are focused in the same point on the recording medium. The multicolor printhead is arranged on a carriage for reciprocal movement backwards and forwards across the recording medium.
Japanese patent application No. 52-67286, published without examination on Oct. 1, 1979, as laid open number 54-2728 to Yamazaki, discloses a single jet, continuous stream type printing head. The printhead ejects a colorless ink which, after charging at a charging station, passes through an accelerating electrode ring and through selectively actuated mist streams of predetermined density and different colors. The mist streams are actuated in response to the recording color desired by the signal for producing each of a variety of colored droplets. Thus, as the charged droplet passes through the one or more colored mist streams, they pick up the desired color prior to impinging on the recording medium.
U.S. Pat. No. 4,382,262 to Savit, discloses method and apparatus for ink jet printing in a plurality of colors on the substrate whereby the apparent drop-on-demand ink jet printing nozzles direct droplets of a liquid to the substrate containing complementary dye components which produce a finished dye at the points of impact of the liquid onto the recording medium. Different ink jet printing nozzles utilize different complementary dye components on the same substrate producing different dyes and thereby different colors. One of the problems solved by this invention is that of nozzle clogging caused by ink composition having high concentrations of dye which produce ink of inherently high viscosity that tend to clog the jet nozzles. In addition, ink jet printing compositions frequently contain dissolved resinous materials to improve adhesion of droplets to a substrate. Dissolved resins exacerbate the viscosity problems and form plugs in the nozzle when the composition dries therein.
U.S. Pat. No. 4,403,228 to Miura et al, discloses a multi-nozzle ink jet printing head. A common air flow chamber is connected with the nozzle to discharge air and ink therethrough to a writing surface. This patent relates to a printhead having a plurality of nozzles in which the combined effects of air pressure gradient and electrical potential gradient are utilized to discharge a controlled number of jet streams of ink droplets. For proper operation of prior art printheads, the liquid nozzle had to be aligned with one to one correspondence with the air-liquid nozzles with a high degree of precision. Because of close tolerances, this alignment is extremely difficult to achieve. One of the stated objects of this patent is to provide a multi-nozzle ink jet printing head of the air and liquid type which can be fabricated without the close tolerances.
U.S. Pat. No. 4,178,597 to Isayama et al, discloses a color ink jet printing apparatus in which an ink ejection head comprises three ejectors for ejecting inks of three primary colors. Each ejector ejects ink of a respective color in response to one of three respective electrical signals. The printhead or ejection head in this patent comprises three ink ejectors for the respective colors which ejects colored ink in superposition on the sheet in response to the three respective electrical color signals. The problem that this patent solves is that erroneous colors are produced at the boundaries of two colors on the sheet where one of the ejectors is shut off and the other ejector is turned on. This problem is solved by circuit means delaying ink ejection from the ink ejector corresponding to the signal which has been initiated. This creates an area at the boundary void of both colors, thus avoiding the production of an erroneous color.
In considering the design of a carriage type ink jet printer capable of more than one color printing, the straight forward approach is complete duplication of all of the printhead functions, that is, the placement of multiple printheads on the carriage equal in number to the number of colors to be printed. A four-color printer, for example, with a single jet for each four colors, magenta, cyan, yellow, and black, would require complete separate systems if designed in accordance with the prior art continuous stream type printheads. Even if the deflection plates and charge tunnels were fabricated as a unit for all four of the jets, each jet would still require an individual nozzle structure and driver with attendant cost and parts as well as alignment.